Optical signal-to-noise ratio (OSNR), as a critical performance index for an optical communication network, is directly associated with bit error rate of an optical signal. A major source of optical noise in an optical transmission line is Amplified Spontaneous Emission (ASE) of an optical amplifier.
The IEC 61280-2-9 standard provides a standard method for determining OSNR in a Dense Wavelength Division Multiplexing (DWDM) system. This method measures a power level of the noise outside the signal bandwidth and interpolates the power level of the inter-band noise to evaluate a power level of the in-band noise in the signal bandwidth. However, this method does not work well in the following two situations. In a first situation, the signal has an inter-band noise and an in-band noise different from each other. For example, when the signal is filtered through such as Reconfigurable Optical Add-Drop Multiplexer (ROADM), the inter-band noise level would be very different from the in-band noise level. In a second situation, the optical signal has a signal spectrum and an inter-band noise spectrum overlapping with each other. For example, the optical signal with a high frequency of 40G/100G has a wideband signal spectrum that overlaps with the inter-band noise spectrum.
A conventional method for in-band OSNR measurement is called as “turning-off”, in which the signal of the channel is turned off to measure the noise in the channel, thereby obtaining the in-band OSNR. Obviously, this method cannot accomplish online measurement of the in-band ONSR. The existing online in-band OSNR measuring methods are mostly based on an assumption that the polarization of the signal differs from the polarization of the noise in the channel. For example, the inherent channel noise is generally non-polarized, while the signal is highly polarized. The US patent application publication US2010/0129074A1 entitled “IN-BAND OPTICAL SIGNAL TO NOISE RATIO DETERMINATION METHOD AND SYSTEM” discloses an online in-band OSNR measuring method which is based on an assumption that the signal light is single polarized light and the noise light is non-polarized light. This method is not applicable for a polarization-multiplexed signal. The US patent application publication US2012/0106951A1 entitled “IN-BAND OPTICAL-TO-NOISE RATIO MEASUREMENT” also discloses an online in-band OSNR measuring method, which requires the signal has periodical power modulation. So, this method is not suitable for an arbitrary signal either.
Therefore, the existing on-line in-band OSNR measuring methods cannot realize a quick and accurate measurement on an arbitrary signal including a polarization-multiplexed signal.